1. FIELD OF THE INVENTION
This invention relates generally to borehole operating tools such as underreamers, and more particularly to an underreamer having three expandable cutter blades engaged by and controlled by a fluid operated piston.
2. BRIEF DESCRIPTION OF THE PRIOR ART
Underreamers are a type of borehole operating tool for use in enlarging an oil well borehole which was initially bored by the drill bit. A typical underreamer includes expandable arms mounted in a housing by suitable hinge pins for movement between a withdrawn or closed position and an open, expanded position. Typically, the expandable arms are moved outwardly by means of a pressure actuated piston mounted within the main bore of the tool housing. The ends of the expandable arms are usually provided with a machined surface or cutting inserts for engaging certain types of formations and cutting a larger hole than created by the drill bit. There are several patents which disclose various underreaming tools.
Simpson, U.S. Pat. No. 4,589,504 discloses a well bore enlarger which has an enlarged section defining blades and cutting arms pivotally mounted in the blades and movable by means of a spring loaded piston between a radially expanded and retracted position.
Muse et al, U.S. Pat. No. 2,822,150 discloses a rotary expansible drill bit having upper and lower cutters pivotally mounted on a main body and connected by a rack and pinion mechanism to a plunger for simultaneously expanding the cutters outwardly from the main body.
Hailey, U.S. Pat. No. 4,809,793 discloses a downhole cutting tool which includes longitudinal bores which are opened to fluid flow when the cutting members are moved to their extended positions. The tool has upper and lower subassemblies connected together through an adapter which establishes an angular offset between an upper and lower pair of extendible cutter members. In both the upper and lower subassemblies, one passage is continuously open to fluid flow at all times and the other passage is initially closed and only opens on predetermined movement of the piston or pistons which open the extendible cutters.
Baker, U.S. Pat. Nos. 2,548,931 and 2,644,673, and Huitt et al, U.S. Pat. No. 3,050,122 disclose underreamers having cutter blades pivotally mounted on a main body and connected by link members to a plunger for simultaneously expanding the cutters outwardly from the main body.
Emanuel et al, U.S. Pat. No. 2,756,968 discloses an expansible well scraper having scraper blades pivotally mounted on a main body and connected by a toothed mechanism to a plunger for simultaneously expanding the cutters outwardly from the main body.
Campbell et al, U.S. Pat. No. 4,565,252 discloses an underreamer or milling tool having simultaneously expansible arms pivotally mounted on a main body. A rotary fluid housing is mounted within each arm and includes a body nozzle for receiving fluid from the body and an expandable arm nozzle for directing fluid into a bore which extends through the arm to provide circulating fluid outwardly of the expandable arm.
The present invention is distinguished over the prior art in general, and these patents in particular by an underreamer tool having a generally cylindrical body with a threaded top end for connection to a tubing string, a threaded lower portion to receive a drill bit, and a longitudinal fluid passageway extending centrally through the body. The body has three flat surfaces formed on its exterior in circumferentially spaced relation to define a generally triangular cross section intermediate the top and bottom ends of the body. A cutter blade is pivotally mounted on each of the flat surfaces. A piston is slidably received in a bore in the upper portion of the body and has central fluid passageway therethrough. Push rod members are connected to the piston inside the body and each has an exterior portion slidably mounted in a slot on the exterior of the body engaging the cutter blades for moving them between a retracted position against the body and a extended position outward therefrom. The piston and push rod members are moveable between an upper position allowing the blades to assume the retracted position and a lower position moving the cutter blades to the extended position responsive to a predetermined fluid pressure acting thereon. A continuous fluid flow path is established from the upper end of the tool to the drill bit at the lower end of the tool regardless of the position of the piston.